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Yet another oven controller

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Hypernova:
Actually bought the parts months ago but didn't get around to building it till this month. The basic idea is to use any dumb oven with minimal modification and be as simple as possible.



Basic functions:
8 phases that can be up to 10 minutes long, adjustable in 10s increments. Each phase has a target temperature up to 360C in 20C increments. When the set time in each phase is reached the controller move on to the next phase until it rolls over to #0 which is the stop phase.The profile is auto saved to eeprom when ever it is changed.
The controller checks the MAX6675 thermocouple decoder at 4Hz rate, if it's lower temperature than the target it turns on the SSR. So it's a pretty basic controller, your average toaster oven doesn't really have that much power to warrant a full PID anyway.
Currently I have a AC zero crossing detection circuit nut haven't really made use of it yet. The SSR is already equipped with ZC function.

Display switches between:
-#- current phase #
### temperature in C, blinks the target temp if you press the "edit" button to edit the profile.
##.# count down to the next phase in seconds, blinks the phase duration if you press the "edit" button to edit the profile.

The second decimal point indicates the state of the SSR, so in the picture below the SSR is ON and temperature is at 199C.

While I have 5 buttons I really only need 4 (Menu/Edit/+/-)




A few thermal shots taken with the imager at work (Fluke Ti10, this thing is awesome!)

The bright spot is the resistor divider for the zero crossing. To show the temperature of the SSR while it's conducting around 5.5A I stuck a piece of silicone thermo pad on it, with the toaster oven I tried it with a heatsink is not really needed.




Major components:
RAC03-05SC AC/DC converter, massive overkill for the job, the only reason I picked it was because it was board mount. Considering that I am only loading it at maybe 40mA any one-hung-low 5v usb wall wart would have served just as well.
G3NE-220T-2-US DC5 Solid State Relay, rated for 20A so I got plenty of head room for bigger ovens.
LCD-S301C31TR 3 digit LCD, dirt cheap and easy to drive. I have character LCDs in my junk box but I wanted something simple.
MAX6675 thermocouple decoder, went EOL by the time I got around to using it, Maxim has released the MAX31855 which only cost half as much...
ATmega8535, I simply filtered for the cheapest AVR that had enough IOs on mouser.

McMonster:
Great post. Earlier today I've been shopping for some components and got so annoyed by all those damned SMD components that I decided to do my own reflow oven. I thought about buing an infrared oven with built in temperature control and adding some simple control circuit. Most of the ovens (dumb or not) are rated for up to about 250 deg. C, I've checked some random solder paste datasheet and it seems that it's more than enough for them. Are there any things that I should specifically watch for when building my own reflow oven?

And what kind of packages I should expect to be able to solder reliably?

Hypernova:

--- Quote from: McMonster on April 30, 2012, 10:01:05 pm ---Great post. Earlier today I've been shopping for some components and got so annoyed by all those damned SMD components that I decided to do my own reflow oven. I thought about buing an infrared oven with built in temperature control and adding some simple control circuit. Most of the ovens (dumb or not) are rated for up to about 250 deg. C, I've checked some random solder paste datasheet and it seems that it's more than enough for them. Are there any things that I should specifically watch for when building my own reflow oven?

And what kind of packages I should expect to be able to solder reliably?

--- End quote ---

In theory if you can control the ramp rate right any SMD package should be doable. IMO the most important thing is a powerful oven that can heat up quickly when you need it to. My oven only has two puny 300W elements and it takes quite a while to heat up.

Now, the reason those oven are "rated" for 250C is due to the thermostat switch inside them to prevent over cooking, although even without bypassing that my oven if left on for more than 5 minutes eventually reaches 300C. Part of what factors into ramp rate is also the heat insulation. El cheepo ovens have no insulation what so ever and a good chunk of the heat is wasted when you really need to keep it in.

If this is your first entry into SMDs I suggest you start with IR or hot air rework stations first, least you solder on some BGA but then have no means of removing it short of rebaking the whole board. A rework station and oven are not mutually exclusive and you really want to have both.

Kremmen:
In the other thread about reflow ovens I showed a photo of my version. Couple of things to consider for proper reflow process:
1. You need sufficient heating power to be able to reach the reflow temperature in a timely manner
2. The soldering cycle should follow a specified temperature curve. The best easily located guide is the Altera FPGA reflow temperature profile. Part of that profile is the idea that the temperature should not exceed liquidus for more than some 30 seconds in order to avoid cooking the components. Also there is the pre-soak phase where you bring the PCB temperature up in order to minimize heat shock.
A simple temperature controller with a single setpoint is not ideal because it won't follow any curve. What you really need is a setpoint sequencer followed by a controller, effectively making the whole a servo system.

Hypernova:
My weak oven is definitely the major weak point. Right now I got most of the phases in 20C/30s steps and that's barley enough time for the oven to reach it before moving on to the next one. I have also tried plugging into 220V and the oven heats up waaaay faster but that has the obvious risk of burning out the heating elements.

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